The system and methods described below enhance the accuracy and effectiveness of cryosurgery of the prostate. Cryosurgery of prostate is an effective treatment for prostate cancer and benign prostate hyperplasia, conditions which affect many men.
The use of cryosurgical probes for cryoablation of prostate is described in Onik, Ultrasound-Guided Crvosurgerv, Scientific American at 62 (January 1996) and Onik, Cohen, et al., Transrectal Ultrasound-Guided Percutaneous Radial Cryosurgical Ablation Of The Prostate, 72 Cancer 1291 (1993). In this procedure, generally referred to as cryoablation of the prostate, several cryosurgical probes are inserted through the skin in the perineal area (between the scrotum and the anus) which provides the easiest access to the prostate. The probes are pushed into the prostate gland through previously placed cannulas. Placement of the probes within the prostate gland is visualized with an ultrasound imaging probe placed in the rectum. The probes are quickly cooled to temperatures typically below -120 C. The prostate tissue is killed by the freezing, and any tumor or cancer within the prostate is also killed. The body will absorb some of the dead tissue over a period of several weeks. Other necrosed tissue may slough off through the urethra. The urethra, bladder neck sphincter and external sphincter are protected from freezing by a warming catheter placed in the urethra and continuously flushed with warm saline to keep the urethra from freezing.
To maximize the effectiveness of the procedure, the entire prostate should be ablated. At the same time, surrounding structures such as the rectum and the neurovascular bundles should not be frozen. The amount of the prostate which is ablated by the cryosurgical procedure depends on the number of cryoprobes used and their placement within the prostate gland. Wong, et al., Cryosurgery as a Treatment for Prostate Carcinoma, 79 Cancer 963 (March 1997), suggests a placement scheme for cryosurgical probes within the prostate. Probes were inserted through the perineal area into the prostate while attempting to keep the probes within 1.8 cm of each other. The systems and methods presented below were developed to assist surgeons in placing the probes as suggested by Wong, or as suggested by others, with the assistance of ultrasound imaging and computer graphics and computer assisted calculations of optimal probe placement within the prostate.